"We must begin by purifying ourselves before purifying others; we must be instructed to be able to instruct, become light to illuminate, draw close to God to bring him close to others, be sanctified to sanctify, lead by the hand and counsel prudently. I know whose ministers we are, where we find ourselves and to where we strive. I know God's greatness and man's weakness, but also his potential." -St. Gregory of Nazianzus
I would only add, and I'm sure Gregory would agree, that our potential can only be actualized by the grace of God, "for it is God who works in you to will and to act according to his good purpose." (Phil. 2:13).